Multigrid electron discharge tube



Dec. 5, 1950 A. J. w. M. VAN O-VERB'EEK ETAL 2,532,369

MULTIGRID ELECTRON DISCHARGE TUBE Filed Feb. 17, 1949 IN VEN TORS.

AGENT.

Patented Dec. 5, 1 950 MULTIGRID ELECTRON DISCHARGE TUBE Adrianus Johannes Wilhelmus Marie van Overbeek and Jan Dorrestijn, Eindhoven, Netherlands, assignors to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application February 17, 1949, Serial No. 76,886 In the Netherlands March 12, 1948 3 Claims.

This invention relates to electric discharge tubes comprising two control grids and an intermediate screen grid, together with an electrode, connected to the screen grid, for collecting the electrons returning from the second control grid.

If the said tubes are used as control tubes, the Variation in the capacity of the control grid when controlled with the use of the second control grid is maintained small owing .to the electrons returning from this grid being collected by an electrode connected to the screen grid. However, this requires the use of a high screengrid voltage since otherwise when the screengrid voltage is derived in the usual way from a potentiometer, this voltage varies to an undue extent in the case of control with the use of the second control grid.

However, a high screen-grid voltage requires the use of a comparatively high control voltage on account of the fact that the control grid must occupy an inclined position to throw the returned electrons. to the electrode connected to the screen grid, since an inclined grid requires the average potential in the grid plane to be positive in order to permit the passage of all electrons. However, since the grid wires must preferably exhibit zero potential or negative potential, the potential distribution along the grid is rendered very irregular, which involves a high control voltage. Consequently, the requirements of high screen-grid voltage and low control voltage conflict with one another.

The present invention provides an electric discharge tube in which the said requirements are greatly conformed to one another so that a low control voltage is obtained even at a high screen'- grid voltage.

According to the invention, in an electric discharge tube comprising two control grids and an intermediate screen grid, which is connected to an electrode for collecting the electrons returning in front of the second control grid, the elongated cathode emits at least one wedge-shaped beam, the apical angle of which is 20 at the most, the distance between the second controlgrid and the screen grid, as measured in the plane of symmetry of the beam, being .at least 1.5 times the width of the active surface of the cathode (measured at right angles to the said plane of symmetry), the perpendicular to the second control grid in the intersecting line of the plane of symmetry and the grid plane making an angle of at the most with the plane of symmetry, and the radius of curvature of the grid at this point in a plane normal to the axis of the cathode being from 1.5 to 4 times the said distance. The electrode having to collect the electrons returned by the control grid is preferably arranged in such manner that at the side to which the beam of electrons on being controlled is defiected, it also constitutes the boundary of the electron beam emanating from the first screen grid.

In the construction described it is ensured that the narrow electron beam is simultaneously reflected and focussed by the control grid and, owing to the large spacing between the first screen grid and the control grid and the small aperture angle of the beam, is nevertheless concentrated on the electrode connected to the screen grid laterally of the beam. The marginal electrons of the beam are subject to the same action as the central beam owing to the curvature of the grid.

Since furthermore the electron beam passed by the control grid is only comparatively narrow, the anode for each beam emanating from the cathode may comprise an elongated strip which is displaced with respect to the plane of symmetry of the beam emanating from the cathode to that side at which the control grid is most remote from the cathode. The electron beam being comparatively narrow, use may be made, instead of a conventional anode, of a secondary-emission output system comprising a single secondary-emission electrode and an anode, in which event the input aperture of the system may be comparatively small. Owing to the small size of the said anode, the tube is excellently suited for use at short waves.

The invention will now be explained more fully by reference to the accompanying drawing, in which:

Fig. 1 shows an electrode system of a tube according to the invention comprising an anode as the output system, and

Fig. 2 shows a similar system comprising a secondary-emission electrode.

In Fig. 1, reference numeral indicates the plane cathode, the upper and lower side of which may emit electron beams. 2 and 3 indicate the first control grid and the first screen grid respectively, which are arranged in parallel with the cathode surface. Two plates 4, interconnected by a transverse plate, constitute on one side the boundary of the electron beam, collecting the electrons returning from the control grid 5. The second screen grid and the anode are designated 6 and I respectively. Of the control grid, the second screen grid and the anode only those parts are shown which are provided in the upper half of the tube. The dimensions of the electrode system follow from the millimeter scale shown below.

In Fig. 2, which is identical with Fig. 1 up to and including the second screen grid, the anode T is substituted by a concave secondary-emission electrode 8, which is faced by a convex anode 9 in such manner that the distance between the two electrodes gradually decreases. In view of the strong load produced by the secondary-emission current, the anode 9 comprises a large cooling fin ID.

The electrode systems are intended for providing the voltage of 250 volts on the first screen-grid, the bias voltage of the control grid of course being volt. The voltage at the second screen-grid is 150 volts, the anode voltage in each case being 250 volts and the voltage of the secondary-emission electrode 8 being 150 volts. When these voltages are applied to the tube, the control voltage is 15 volts at the most.

What we claim is:

1. An electric discharge tube comprising a cathode having an axis and an active surface of given width for generating a wedge-like beam of electrons, said beam having a plane of symmetry, first and second control grids in spatial relationship with said cathode, said second control grid having a grid plane and a radius of curvature, a screen grid interposed between said first and second control grids, and an electrode member in electrical connection with said screen grid, and in which tube the apical angle of the wedge-like beam generated by the cathode is less than about 20, the spacing between the second control grid and the screen grid as measured in the plane of symmetry of the electron beam being at least one and one-half times the width of the said active surface of the said cathode, the perpendicular to the second control grid in the intersecting line of the said plane of symmetry and said grid plane making an angle of approximately 15 at most with the plane of symmetry, while the radius of curvature of the grid in a section at right angles to the axis of the cathode at the point of the perpendicular to the said intersecting line is equal to one and one-half to four times the said spacing.

2. An electric discharge tube as claimed in claim 1 in which for each electron beam generated by the cathode the said tube comprises an anode in the form of an elongated strip which has a width of not less than two and one-half times the width of the active surface of the oathode, the said strip being located asymmetrically with respect to the said plane of symmetry of the electron beam and at the side of the control grid which is most remote from the first screen grid.

3. An electric discharge tube as claimed in claim 1 in which for each electron beam generated by the cathode the said tube comprises a secondary-emission output system having a secondary-emission electrode having a surface concave with respect to said electron beam, and an anode having a surface convex to said secondaryemission electrode, the said anode being spaced from the said secondary-emission electrode and forming an aperture therebetween the said spacing decreasing in the direction of the movement of electrons from the said cathode, and the said aperture being positioned asymmetrically with respect to the plane of symmetry of the electron beam generated by the cathode and having a width which is two and one-half times the said active surface of the said cathode.

ADRIANUS J OI-IANNES WILHELMUS MARIE VAN OVERBEEK. JAN DORRESTIJN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,138,228 Ericksen Nov. 29, 1938 2,460,062 Charton .Jan. 25, 1949 

